Since the present invention finds particular utility in two-cycle xerographic devices, making use of a single magnetic brush developer to sequentially develop and then clean the photoconductor, a brief description of this art is in order.
The use of a single magnetic brush developer station to first tone or develop the photoconductor, during a first cycle of the photoconductor through the developer, and to later clean that same photoconductor area, as that area passes through the developer during a second cycle, is well known in the art.
U.S. Pat. No. 3,647,293 is believed to be the pioneer patent disclosing combined developing/cleaning by the use of a magnetic brush developer. In this patent, it is recognized that it may be necessary to change the developer's development electrode bias, depending upon the mode (developing or cleaning) in which the developer is then operating. The need to change this bias is said to be a function of photoconductor voltage during the cleaning cycle, and, if the photoconductor is adequately discharged, the same bias voltage should be adequate for both functions.
This is also the general teaching of U.S. Pat. No. 3,637,306.
U.S. Pat. No. 4,205,912 is of interest in that a photoconductor drum makes two revolutions or cycles in order to produce one copy. In the first cycle, the photoconductor is charged negatively, and the original document's negative latent image is toned by positively charged toner, as this image passes through a magnetic brush developer which is supplied with a negative development electrode bias voltage. This photoconductor-carried toner is then transferred to paper at a negative corona transfer station. In the second photoconductor cycle, the photoconductor and its residual toner (the toner which did not transfer to paper) are subjected first to a preclean erase lamp and then to a positive polarity preclean corona. As a result of this latter corona, any residual toner which was charged negative at the transfer station is charged back to its original positive state. This positive residual toner now enters the magnetic brush developer (now operating as a cleaning station), and the positive toner is cleared from the photoconductor due to the developer's negative bias voltage.
U.S. Pat. No. 4,087,170 also discloses this type of two-cycle device.
Another two-cycle arrangement whereby a magnetic brush developer affects both development and cleaning is shown in U.S. Pat. No. 4,142,165. In this patent, the same development electrode bias voltage is used for both development and cleaning. However, a scavenging electrode in the form of a roller is provided to electrostatically remove a predetermined amount of toner from the magnetic brush prior to the toner's entering the cleaning nip at the photoconductor. This magnetic brush, now somewhat depleted of toner, is said to better clean residual toner from the photoconductor.
The present invention makes use of what is known as a multiple brush magnetic brush (i.e. multiple magnetic brush rollers) developer. Such developers per se, are known in art and are exemplified by U.S. Pat. Nos. 3,805,739; 3,985,099; 4,266,868 and 4,279,942.
In U.S. Pat. No. 3,805,739 the development electrode bias voltage is capable of a magnitude change in order to control copy print quality.
In U.S. Pat. No. 3,985,099 the magnetic polarity alternates from roller to roller.
In U.S. Pat. No. 4,266,868 each roller's nonmagnetic outer sleeve and inner magnet assembly is capable of independent rotation.
In U.S. Pat. No. 4,279,942 the roller's inner magnet assembly is capable of position-adjustment, whereby the toner's electrical conductivity is said to change, i.e. becomes higher as the toner is packed together more tightly.
The aforesaid patents are hereby incorporated by reference as illustrative of the state of the xerographic art, the two-cycle process and multiple roller magnetic brush developers.
The present invention makes use of development electrode bias voltage switching, as a function of the ever-changing position of a moving xerographic photoconductor.
The IBM TECHNICAL DISCLOSURE BULLETIN, December 1981, at pages 3782 to 3784, describes switching a magnetic brush developer's development electrode bias voltage as a function of the position of the drum seal of an incrementing photoconductor drum, to prevent carrier beads from being carried out of the developer.
The IBM TECHNICAL DISCLOSURE BULLETIN of April 1981, at pages 5103 and 5104, describes a multiple-brush developer where adjacent rollers have different levels of bias voltage, to thereby effect different latent image development characteristics at each roller.